Very high frequency variable inductor



8- 5, 1950 w- J. POLYDOROFF 2,513,930

VERY HIGH FREQUENCY VARIABLE INDUCTOR Filed Oct. 16. I946 Patented Aug. 15, 1950 UNITED STATES PATENT OFFICE VERY HIGH FREQUENCY VARIABLE INDUCTOB Wladimir J. Poiydorofl, Glencoe, Ill.

Application October 16, 1948, Serial No. 703,522

I Claim The present invention relates to very high frequency circuits of tunable or adjustable type, such as employed in frequency modulation, television or communication circuits.

More particularly the invention relates to the tunable inductances of the type where inductance variation is obtainable by a movable or slidable magnetic core.

In tuned circuits in order to maintain a certain L/C ratio the inductance becomes exceedingly small at high frequencies of the order of 40-200 megacycles if a certain lump capacity is allowed in the circuit for the frequency stability with operation. The number of turns is necessarily reduced to a few turns and the turns must be widely spaced. This later expedient is essential when the coil is tuned by a movable iron core since the movement must be large enough to spread the tuning rang over a certain length of tuning dial and a mechanical gear associated with the movement. In actual practice a movement from 1 to 1 is required to make this tuning practical, and also to correspond with a similar movement already in use in the lower frequency bands.

Therefore if an ordinary coil inductance is employed the turns must be spaced over the entire length of the core, when the latter has entered the coil. With, for instance, a four-turn-coil spread over the length of an inch or more the spacing between the turns is increased and the magnetic linkage is greatly reduced.

Successful core materials now in use for the frequencies between 40 and 200 me. have their full toroidal permeability ranging from 3.5 to 6 or very much reduced in order to maintain permissible Q( L/R) in the coil at those frequencies.

Reference is now made to my publication in periodical Radio of November, 1945 which establishes the relation between the toroidal (full) permeability and the effectiv permeability of a cylindrical core. If we assume a practical core length of 1%" and its diameter /4" we arrive to a length to diameter ratio as 5 to 1 which yields an effective permeability of from 2.5 to 4.0 which is only realizable if the core is fully and uniformly wound with a wire without a spacing. AllowcreasetherangebuttheQofthecoiiwillbe severely impaired.

ing four turns of wire spread over the entire length the linkage between the turns is weakened to a resultant permeability reduction of about which is further reduced when a necessary spacing between the core and the coil is introduced. Finally the range of the tuning becomes too small for practical application. The employment of material of higher permeability will into frequency range.

The present invention overcomes the diiiiculties explained above and provides a novel and eillcient device enabling the very-high-frequency circuits to be tuned over certain bands of frequency in the region between 50 and 200 mega cycles.

One object of the invention is to provide an eiiicient and stable in operation very-high-frequency circuit tunable over a desired frequency range by variation of its inductance element and having conveniently large fixed capacitive element.

Another object is to provide a variable inductance device of low enough inductance value yet capable of being altered b the presence of a magnetic core through a desired range. while the displacement of the core is of appreciable and of convenient length.

Still another object is to provide a variable inductance having high value of its electrical quality Q throughout the entire tuning range.

Still another object is to provide a variable inductor of simple and rugged construction which can be easily and uniformly reproduced in quantity production.

All of the above objects together with others which follow form the present invention will be readily understood if reference is made to the drawings, in which:

Figure 1 shows a conventional coil and core.

Figure 2 shows the circuit and coil of the present invention in its simplest form.

Figures 3 and 6 show the preferred modification of the invention.

Figure 4 shows a typical very-high-frequency circuit embodying the present invention and Figure 5 shows very-high-frequency oscillator circuits of which the present invention forms a part.

Referring now to the Figure 1 a solenoidal coil I is shown togetherwith a movable ferromagnetic core 2 which may slide in and out of the coil. As has been stated the invention is for the veryhigh-frequency circuits in the region from 50 to 200 me. and the values discussed now are related to a mean frequency of about me. At that frequency in order to resonate the circuit with an appreciable capacity of, say, 25 in, the inductance will be of the order of 0.1 microhenry. Taking as practical dimensions the diameter of the core about and its length 1%" we will have a total displacement of 1 to cover the desired This means that the soienoidal coil dimensions will be: length approximately 1%" and inner diameter slightly over Such a coil tor a given above inductance will have between 2 and 3 turns of wire 16 or 14 gauge spread over the entire range. With a core material used ior these frequencies and with the reduced linkage between the turns very small inductance variation results, insuflicient to cover a desired frequency range 01', say, '75 to 100 me. 'For many circuit considerations the coil leads should be kept as short as possible in order to comp. :te the circuit with a fixed capacity as shown in Figure 4 and to terminate the circuit with a radio tube element. In the construction shown one lead, for example 3 may be made as short as practical, while the other lead 3a must of necessity be longer than the length of the coil. Such a lead wire presents an additional inductance which is appreciable compared with the total inductance of the circuit (0.1 h.) and may be as high as or of the total inductance. This portion is not variable and is in series with the variable portion of the inductance. Consequently, if core produccs a certain inductance variation in the coil, for instance Ite=2 the final inductance variation will be reduced to 1.8 or 1.6 because of the fixed inductance of the leads. To compensate for this drop in effective permeability the diameter of coil may be reduced in order to increase number of turns) which results in the drop of Q; or several strands are laid parallel which increases the capacity of the circuit. thus reducing the lump value of C and stability of operation. Both expedients are used together resulting in costly and ineflicient construction, which in addition is hard to duplicate.

Referring now to the Figure 2 a similar and of larger dimensions coil 3 can be used to the advantage if the detrimental effect of leads is eliminated. In this figure coil 3 has one lead 4 very.

short and connected to a fixed condenser 5 and a tube 6 while the other long lead I is threaded exactly through the center oi. the coil. Actual measurements indicate that such position of the leads decrease the total inductance of the coil per se. Thus in one instance 01' measurements the coil with the lead on the outside, as per Figure 1, had its inductance measured by resonance method to be: 1.3 ah. and when the lead was brought in the coil coaxially with its axis the inductance dropped to 1.15 h. In another instance of the coil of Figure 3 the air inductance of the coil is 0.87 h. with the lead in conventional manner and the same coil inductance dropped down to 0.67 when the lead was brought coaxially through the coil. In both coils in both instances the high value oi Q remained whether the lead 1 is inside or outside of the coil. Such considerable reduction in the inductance value by the mere change of position of the lead could not be explained by the interposition of the copper in the center of coil, which would immediately manifest itself in the drop of Q.

To verify this statement, a separate wire of the same length and thickness was positioned in the middle of conventional coil of Figure 1 with the resultant drop of Q of less than 1% and hardly appreciable (less than /2%) drop of inductance.

The phenomenon is very important for the present invention and is therefore explained in the following manner, without, however, giving a definite proof. The inductance of the coil is composed of incremental inductanccs of each turn adding together plus a mutual inductance increments from turn to turn also adding together. The mutual inductance, which I have called "linkage" between the turns in my above mentioned specification, is reduced by the presence of the return wire passing through the magnetic center of the coil, causing the de-magnetising effeet on the total inductance. Thus by this new expedient we are able to construct geometrically larger coils for a given inductance without detri ment to its electrical properties in those cases where the inductance of the coil leads is comparable with the coils inductance, i. e. at veryhigh-frequencies.

If now the low loss ferromagnetic core is insorted in the middle of the coil a very much greater inductance variation, or Fe, is obtainable. In the case of a conventional coil the [Le was found to be 1.8 while when the core is placed in the center of coil of Figure 2 with the return lead passing through the center hole in the core 8 [Le increased to 2.4 which is already a considerable improvement; it is still further amplified in the coil construction of Figure 3 wh re original inductance increase in terms of 23 was only 1.4 and when return lead and the core are placed in accordance with the invention the la increased to 1.85 or 30%.

This unexpected increase in inductance variation can be explained as being caused by three difierent actions all in the same direction:

1. The de-magnetising efiect of the lead wire is shielded out" or nullified by the presence of the core surrounding the wire.

2. The inductance of the lead per se is increased by the presence of the core surrounding the lead; this is in accordance with the theory of the inductance of a straight wire and was seprately verified by placing the core around one lead of conventional coil of Figure 1.

3. The inductance of the coil turns and the linkage are increased by the presence of the core in the coil in the usual manner. I

All three efiects acting in the same direction cause a very much greater inductance variation while maintaining Q of the coil.

The invention permits further decrease in the coils dimensions and number of turns, thereby permitting the use of larger capacity for a circuit of given frequency range, thus assuring greater stability of operation. It is known that when a tube is connected to the circuit its input capacity is appreciable and varies with temperature and plate voltage fluctuations. The lesser portion cf the total capacity is in the tube, the greater will be the stability of the circuit.

The Figure 3 shows a modification of Figure 2 of the present invention in that the inductance is further decreased and the length of coil increased while the effective permeability with high quality low permeability core is of sufficient magnitude to cover a television or frequency modulation band of frequencies, as now assigned.

A very sturdy construction of the coil 9 is obtained by taking a hollow tube of highly conductive material, having its inner diameter of and cut to the length of about 1 /2". The wall of the tube is slotted diagonally with a straight or circular saw so that a helixidal slot is formed around the most of the length of the tube. Thus 2 to 4 turns of coil is formed depending on the pitch of the slot. One end of the coil has a soldered lead 4, which leads directly to the capacitor and the grid of the tube, being as short as practice permits. The other, long leads 1 is soldered to the other end of the coil and bent inward coaxially with coil. The values of inductance L in air have been already compared and discussed; the measured Q of such coll at 100 me. is of the order of 150 to 200, usually slightly less than in conventional coils. The core 8 for tuning this coil is composed of low permeability low loss material, the original synthetic magnetite powder, being known under the trade name of IRN 9 or IRN 8 the powder being compressed with Bakelite or other suitable binder and after baking forms a solid core material of toroidal (full) permeability from 3.5 to 5.

The core forms a cylinder 1 /2 long of under diameter with a longitudinal slot to accommodate the bend of the inner wire, the width of the slot depending on the thickness of the center lead wire. It has been found that the increase of the thickness of the center lead wire produces an increase in efi'ective permeability but the slot being wider reduces the magnetic content of the core. There seems to be an optimum value of the thickness of the center lead between 1 and 2 millimeter diameters.

The core thus enters the coil from the "back end of the coil and the maximum inductance is realized when the core is fully in the coil.

In view of uneven inductance variation with the core movement it may be found useful to modify the slot in the tubing so as to produce variable pitch of the turns of the coil,- in other words making some turns wider in their axial length than the other turns.

Obviously the same construction can be realized by winding a flat ribbon of copper or silver around a supporting insulating tube to substantially produce the same efiect. Practice shows that the core materials having high internal reslstivty may be used directly in the metal tube forming a coil without detriment to its Q although while moving the core for tuning it may produce scratching noises, so that a thin layer of insulating coating such as enamel on the wire may be preferable.

The same construction of the coil can be applied to higher frequency, such as 200 me; the inductance of the coil should be further reduced (in order to provide enough fixed capacity in the circuit) and as a final shape the coil may have just one turn by diagonally slotting the copper tubing once across its periphery, or leaving the ends of the tubing intact so as to provide solid rigid ends, the leads being soldered as per Figure 6. When the length of the coil is reduced to 54" the inductance in air becomes of the order of .025 ah, with a core of same length the tuning ran e becomes 160-200 me. while a permissible Q 100-150 is maintained. The effective permeability is therefore 1.55 in this extreme case of one turn coil. Naturally larger initial inductance, if permissible, will require more turns and the device will produce wider range of tuning.

The ordinary coils cease in their utility above 100 me. after which frequency transmission line (coaxial cable) Lecher wires or cavity resonators are used. The invention therefore permits the useful extension of coil and its inherent advantages to higher frequencies.

Amongst the advantages foremost is flexibility of design features, unobtainable with other mentioned systems. A coil permits an inductive coupling of certain elements of the circuit. Thus in the case of coupling an antenna a single opencirculted turn may be connected to dipole antenna. Such circuit is shown in Figure 4 where the coil of Figure 3 with its movable core is used to tune the circuit II. A single turn ll placed in inductive relation to the coil I serves as coupling means of a dipole II.

It is known that a tube 6 when connected directly across the tuned circuit, such as indicated in Figure 2 produces a very high damping effect on the circuit, reducing the gain of the antenna circuit to a very considerable amount. Thus in a circuit having Q=l00 the eifect of the shunt equivalent to the tube load may reduce Q to a value of 25. In my U. S. Letters Patent 2,158,251 in tuning high frequency circuits I have advocated the use of a variable portion of the tuning inductance to apply a certain reduced voltage to the grid of the tube; with the present design of very-high-frequency coil I am now able to apply a certain portion of the total voltage to the grid of the tube and if I choose the point of junction of inner wire with the coil I will get a substantially proportional voltage delivered the grid, regardless the core position. Such arrangement is indicated by connection 15 of Figure 4.

In the very-high-frequency oscillator the use of such inductor permits a third connection to be taken as a tap from the coil or at the junction" point as now shown on Figure 5. This considerably simplifies the construction of the oscillator circuit which will now have a single capacitor across the inductor, eliminating two more condensers used in the Colpitts oscillator as has been advocated for permeability tuning andior transmission line and cavity resonator tuning.

The invention is not limited in its construction or application to the description and figures of the present specification but can be advantageously applied to any other inductive devices employed in very high frequency circuits where inductive tuning or adjustment is practical, or where inductive coils of larger dimension and larger number of turns may be reduced in their initial inductance value by the expedients herein described. In this specification the definition "very-high-frequency applies to the recently allocated band of frequencies from -300 me.

What I claim is:

1. A variable inductor for very high frequency range comprising a helical portion wound with several turns of a flat conductor and a linear portion located coaxially with said helical portion and serially connected with one end of said helical portion, a movable high frequency ferromagnetic cylindrical core having a slot throughout its length to accommodate said linear portion, said core moving into said inductor from the end of the connection between two said portions to thereby simultaneously increase the inductance of both portions of the inductor.

2. A variable inductor for very high frequency range com-prising a helical portion wound with several turns of wire to form a coil and a linear portion serially connected with one end of said helical portion and located coaxially with said helical portion inside of said coil, a movable high frequency ferromagnetic core having a slot throughout its length to accommodate said linear portion, said core when moved from the end of the junction of said two elements simultaneously increases the inductance of both said portions.

3. A variable inductor for very high frequency range comprising a helical portion wound with several turns of wire to form a coil and a linear portion serially connected with one end of said helical portion and located coaxially with said helical portion inside of said coil, a movable high frequency ferromagnetic core having a slot throughout its length to accommodate said 7 linear portion, said core when moved from the end oi the junction 01' said two inductive portions simultaneously increases their inductance values and a separate additional connection terminal from the point or said junction.

4. A variable inductance for very high frequency circuit including a movable cylindrical core of low permeability and two inductance elements serially connected and placed coaxially with said core. one of said elements being linear and the other in the form 0! a helix, said core having a radial cavity extending lengthwise of the core to simultaneously engage both of said elements and to produce inductance variation in additive sense.

5. A variable inductance for very high frequency circuits including a movable elongated core of low permeability, a linear element of low inductance and another return element of considerably higher inductance, both elements being connected in series and in additive sense,

8 said linear element being located inside of said return element, said core being provided with a cavity extending lengthwise in order to engage simultaneously both said elements and to produce inductance variation in both elements.

WLADIMIR, J. POLYDOROFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Vaughn July 20, 1948 

